Optical harmonic generation is well known in the art. Intracavity second harmonic generators are known. See for example, J. M. Y. Yarborough, et al., “Enhancement of Optical Second Harmonic Generation by Utilizing the Dispersion of Air”, Vol. 18, No. 3., Applied Physics p. 70–73. Third harmonic generators are also known in the art. Generally, third harmonic generation requires the generation of a second harmonic beam. External third harmonic lasers, where a second harmonic nonlinear crystal and a third harmonic nonlinear crystal are located outside the cavity, are known in the art, see for example U.S. Pat. No. 5,835,513. Intracavity third harmonic lasers are also known, See: U.S. Pat. Nos. 5,898,717. Fourth and fifth harmonic generators are also known. It is desired to provide improved external third, fourth and/or fifth harmonic generation efficiency.